GPS is a Global Navigation Satellite System (GNSS) that comprises a collection of twenty-four earth-orbiting satellites. Each of the GPS satellites travels in a precise orbit about 11,000 miles above the earth's surface. A GPS receiver locks onto at least three of the satellites to determine its position fix. Each satellite transmits a signal, which is modulated with a unique pseudo-noise (PN) code, at the same frequency. The GPS receiver receives a signal that is a mixture of the transmissions of the various satellites that are visible to the receiver. The GPS receiver can detect the transmission of a particular satellite based on corresponding PN code. For example, the detection may comprise correlating the received signal with shifted versions of the PN code for that satellite to identify the source satellite for the received signal and to achieve synchronization with subsequent transmissions from the identified satellite.
When the GPS receiver is powered up, it typically steps through a sequence of states until it can initially determine a navigation solution comprising position, velocity and time. Afterwards, the satellite signals are tracked recurrently to update the position calculations. Precise orbital information, known as ephemeris or ephemeris data, is transmitted by each satellite for use in calculating the navigation solution. Ephemeris or ephemeris data for a particular satellite may be decoded from orbit data once the GPS signal has been acquired. Each satellite broadcasts its own ephemeris data, the broadcast lasts for 18 seconds, repeating every 30 seconds.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.